Polygonal hotspots works perfect in html5 : https://krpano.com/docu/xml/#hotspot borderwidthhover and fillalphahover are old syntax I guess ? You can use onover and onout for that
right, the 'borderwidthhover' and 'fillalphahover' settings are very very old ones and were only available in Flash viewer.
When the HTML5 viewer was developed there were already onover/onhover/onout events and the tween action for changing variables for some times and they were the preferred way for changing things (because that allows much more control), so these settings were never ported to HTML5 and be deprecated.